The present invention is directed to magnetic bearings and to magnetic bearings in general, and to the use of magnetic bearings as configured in bearingless drives.
Broadly, a magnetic bearing supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact; for example, they can levitate a rotating shaft of the rotor of a motor and permit relative motion without friction or wear. Magnetic bearings are used in lieu of a rolling element or fluid film journal bearings in some high performance turbo-machinery applications. Since there is no mechanical contact in magnetic bearings, mechanical friction losses are eliminated. In addition, reliability can be increased since there is no mechanical wear. Following are principles of operation, and examples, of conventional magnetic bearings.
FIG. 11 schematically represents a conventional magnetic bearing arrangement for a rotary device. A motor 1102 connected to a shaft 1104 is suspended by magnetic bearings 1106a, 1106b. Each bearing 1106a, 1106b generates radial forces in the x- and y-directions. A thrust bearing 1108 is optionally provided for positioning in the z-direction.
The motor 1102 is a conventional brushless motor comprising a stator element 1112 and a permanent magnet rotor element 1114. The stator 1112 comprises a set of phase coils which are energized in a specific manner to produce rotational torque about the z-axis. The construction of stators and rotors are well understood and need no further elaboration. An example of motor 1102 is a three-phase motor, where the stator 1112 comprises three phase coils. A suitable three-phase voltage source and a controller serve to energize the phase coils of the stator 1112 which magnetically interact with the rotor 1114 to produce rotational torque.
The magnetic bearings 1106a, 1106b each typically comprises four coils (see FIG. 12) arranged in respective bearing stators 1122a, 1122b. The bearing rotors 1124a, 1124b comprise a magnetic material. For each bearing stator 1122a, 1122b, two coils are arranged on the x-axis on opposing sides of the shaft 1104, and two coils are arranged on the y-axis on opposing sides of the shaft. Differing amounts of current are made to flow in each coil in order to affect the radial position of the shaft 1104 in the x-y directions. For example, when current flows in the coils, opposing magnetic forces are generated which act on the rotor 1124a. A radial force in the x-direction can therefore be generated by creating a difference in the magnetic forces generated by the x-axis coils; likewise for the y-direction.
Coil currents in the bearing stators 1122a, 1122b are regulated by suitable power and control circuitry. For example, a single phase voltage source can be provided for each coil (a total of eight for both magnetic bearings 1106a, 1106b).
Further detail of one of the bearing stators 1122a, 1122b is illustrated in FIG. 12 along with additional electronics detail. Referring to FIG. 12 then, a bearing stator 1222 is shown in cross-section and reveals an example of the arrangement of coils which comprise the bearing stator. Coils 1226x1, 1226x2 constitute the x-axis coils and coils 1226y1, 1226y2 constitute the y-axis coils. In the example shown, each coil (e.g., coil 1226x1) comprises a pair of coils arranged in a horseshoe configuration.
Conventionally, the radial position of the rotor 1224 is sensed by a series of gap sensors 1202. Output from the sensors 1202 feed into suitable gap sensor electronics 1204 to produce a usable signal for a controller 1206. The controller 1206 drives power amplifiers 1208 (two are illustrated) to supply sufficient current to the coils 1226x1, 1226x2, 1226y1, 1226y2 to energize the coils in a manner that positions the rotor to a desired position in the x-y plane.
Typically, the controller 1206 and/or the gap sensor electronics 1204 process signals in the digital domain. In other words, the position signals output from the gap sensors 1202 (usually analog) are processed as digital signals by the controller 1206 to determine the amount of correction in the x-direction and in the y-direction that is needed to place the rotor in a desired position. The x- and y-direction correction data are then converted back to analog signals so that the power amplifiers 1208 can produce suitable drive currents to energize the coils 1226x1, 1226x2, 1226y1, 1226y2 appropriately.
In the magnetic bearing arrangement shown in FIG. 11, the magnetic bearings 1106a, 1106b are elements separate from the motor 1102. However, the magnetic bearings can be incorporated in the construction of the motor in an arrangement referred to as a “bearingless drive”.
FIG. 13 shows an example of a bearingless drive. The bearingless drive shown in the figure comprises two bearingless motor units 1302a, 1302b. Each bearingless motor unit 1302a, 1302b comprises, respectively, a stator element 1322a, 1322b and a rotor element 1324a, 1324b. Although not explicitly shown in the figure, each stator element 1322a, 1322b comprises two set of windings; there is one set of windings (phase coils), called the motor windings, for torque production and there is a separate set of windings, called the suspension windings, for rotor suspension. The motor windings are energized by a source of drive currents to produce rotational torque. The suspension windings are energized by a separate source of drive currents to produce radial forces for positioning in the x-y direction (i.e., the radial direction).
The motor windings in each stator element 1322a, 1322b are connected in parallel. A generator produces drive currents to energize both sets of motor windings for rotary operation. Each of the suspension windings in each stator element 1322a, 1322b, on the other hand, is energized by its own generator in order to provide independent suspension control by each suspension winding.